Method for solidifying while rubbing the solid-liquid interface

ABSTRACT

The microstructure and the impurity distribution in a solid which has been directionally solidified from a melt or solution can be strongly influenced by rubbing the solid-liquid interface during solidification. If the solid has two or more phases, a major effect will be the production of a fine-grain microstructure. The grains in addition to being small will be roughly equiaxial and not possess the usual columnar, dendritic and substructures. If the solid is single phase, the rubbing should break up the diffusion layer in the liquid, which is, typically, enriched in one or more of the constituents, and the equilibrium segregation coefficient between the solid and the liquid should be realized in normal freezing. Fine-grained microstructures have recently become of interest in the field of superplasticity and an important application of normal freezing of a single-phase material is in the desalination of water.

United States Patent [72] lnventor William G. Pfann Far Hills, NJ. [211 App]. No. 851,656 [22] Filed Aug. 20, 1969 [45] Patented Nov. 16, 1971 [73] Assignee Bell Telephone Laboratories, Incorporated Murray Hill, NJ.

[54] METHOD FOR SOLIDIFYENG WHILE RUBBING THE SOLID-LIQUID INTERFACE 6 Claims, 3 Drawing Figs.

52 US. or 23/295, 23/301 SP. 75/135, 164/82 [51] Int. Cl B0ld 9/00 [50] Field of Search 148/1.6; 23/273, 295, 301 SP; 164/51, 71, 82, 260, 251; 264/68, 310, 330, 331, 60; 62/58 [56] References Cited UNITED STATES PATENTS 1,892,806 l/l933 Pedersen 23/273 SP 3,208,] 12 9/1965 Scribner 164/260 LOAD MOTION GENERATOR f 3,470,039 9/1969 Goundry et al. 23/273 SP 3,321,282 5/1967 Schneider et al. 62/58 3,494,139 2/1970 Shapiro et a] 62/58 Primary ExaminerDelbert E. Gantz Assistant ExaminerG. J. Crasanakis AnomeysR. J. Guenther and Edwin B. Cave ABSTRACT: The microstructure and the impurity distribution in a solid which has been directionally solidified from a melt or solution can be strongly influenced by rubbing the solid-liquid interface during solidification. If the solid has two or more phases, a major effect will be the production of a finegrain microstructure. The grains in addition to being small will be roughly equiaxial and not possess the usual columnar, dendritic and substructures. If the solid is single phase, the rubbing should break up the diffusion layer in the liquid, which is, typically, enriched in one or more of the constituents, and the equilibrium segregation coefficient between the solid and the liquid should be realized in normal freezing. F inc-grained microstructures have recently become of interest in the field of superplasticity and an important application of normal freezing of a single-phase material is in the desalination of water.

l li l iil i PATENTEnuuv 16 I971 3. 620.686

FIG.

MOTION GENERATOR INVENTOR n. G. PFAN/V BACKGROUND OF THE INVENTION 1. Field of the Invention The invention pertains to the production of a solid from a melt or solution by directional freezing.

2. Prior Art The control of the crystalline microstructure of metals and alloys has always been of concern to. metallurgists'Recently, some interest has focused on the phenomenon of superplasticity. It has been found that some alloys when'suitably treated have been able to sustain elongation by a factor of times to times before failure. This has led to the investigation of new metal fabrication techniques such as the blowing of metal bubbles. The phenomenon has been linked with the possession of a fine-grained crystalline microstructure..-Such microstructures have been produced by such methodsas extrusion,.hot working during cooling, and cold working, all to reductions of the order of at least five times. These processes require the input of large amounts of mechanical-energy.-In some alloy systems such microstructures can be obtained by heat treatment alone such as the Zn-Al system in which a range of alloys is single phaseat elevated temperatures and two phase at room temperature. The phase transformation upon cooling produces the desired fine microstructure. Ithas been found to be difficult to produce fine-grained microstructures in single-phase materials because it is the presence of the second phase which limits grain growth.

Normal freezing is an important process in material purification because of the fact that during solidification thereis, typically, a large difference betweenthe concentration-of an impurity in the liquid and in the solid. If, for instance, the solid contains a much lower concentration than the liquid, the impurities are swept ahead of the advancing liquid-solidinterface. This produces an artificially high concentration of the impurities in the diffusion layer in the liquid at the interface. This causes the inclusion in the solid of more-ofthe impurity than would be deduced from the consideration of the equilibrium distribution coefficient. It hasbeen found that this diffusion layer cannot be broken up by the agitation or stirring of the liquid alone.

SUMMARY OF THE INVENTION of the boundary layer allows a much closer approach to the equilibrium distribution of any dissolved substances between the liquid andthe solid. In.-the case of single-phase materials, the application of the invention does not necessarily lead to a material decrease in grain size since grain growth is not-inhibited by the presence of a second phase. However, in certain cases (e.g. Bi), the microstructure may be modified in that the grains, although only somewhat smaller, are still roughly equiaxial and not elongated in the growth direction by the lO-to-Al ratios which are typical of directionally solidified materials. This widely applicable method for producing finegrained alloys requires very little mechanical energy 'since it operates very near the melting point of the material where the material is weak. In addition to alloy systems, where interest focuses on the new fabrication techniques implicit in the phenomenon of superplasticity and on the control of strength, magnetic, and superconducting material properties, the invention is applicable to a wide range of nonmetallic inorganic and organic multiphased systems. As applied to single-phase systems the use of the invention will lead to more effective segregation of impurities during normal freezing. The purpose of this normal freezing might be the purification of the body of the solid such as in the desalination of water or the concentration of the impurities in the liquid or at the end of the solid for easier identification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective viewpartly in section of an exemplary solidification apparatus;

FIG. 2 is a perspective view of a wedge rubbing body; and FIG. 3 is a perspective view of a rotary file rubbing body.

DETAILED DESCRIPTION OF THE INVENTION Classes of Materials The invention described in this disclosure is applicable to a wide range of materials,'systems of materials and combinations of materials. Among them are multiphase metallic and nonmetallic systems and single-phase materials with or without minor constituents or impurities. The multiphase metallic alloys include, for example, eutectic systems such as Pb-Sn, Sn-Zn, AlqSi, and Ag-Cu, and noneutectic systems such as the phosphor bronzes and the Al-laase, Ni-base and Zn.-base casting alloys. Among the nonmetallic organic and inorganic multiphase systems are azobenzene-benzyl and carbon tetrabromidehexachloroethane. The single-phase materials to which this invention is'applicable include all crystalline materials which can be grown from a melt or solution. Among the metals are Pb, Sn, Bi, and Ge, and semiconducting compounds, such-as InSb. Among the many organic compounds are 'benzoic acid and acetanilide. One of the more important inorganic systems is water containing salt in solution..

Mechanical Details An apparatus embodying the disclosed invention-must possess some means for producing directional solidification, a nonreacting solid rubbing body and a means for producing the rubbing motion. One possible method of producing directional solidification for a suitable material system is the introductionof enough heat in one part of the apparatus to maintain the material in a molten state and the removal of that heat in another part-of the apparatus. Thedirectional solidifiplane of the liquid-solid interface or vibratory motion taking place perpendicular to the liquid-solid interface in combination with'one of the other motions. Some means must be provided to maintain the rubbing 'body in contact with the advancing solid-liquid interface. Compound structures can be obtained, if desired, by providing this rubbing action over only part of the surface of the solidifying body..

Examples FIG. 1 shows an exemplary apparatus which includes a heater 15 which may provide thermal energy directly or may provide high-frequency electromagnetic energy if the solidifying liquid is a conductor of electricity or contained in a conductor. Here the thermal energy is removed in a coolant bath 11. The motion of the rubbing body 14 which is depicted as a wire brush is produced by a motion generator 16 which may be a simple rotator. The shaft and rubbing body 14, l8'are free to move up and down and the rubbing body is maintained in contact with the solid-liquid interface by means of load 17. In order to produce directional solidification, the containing tube 19 is lowered at a controlled rate through the heater 15 into the coolant bath 11. A number of trials of this invention TABLE 1.Pb-Sn SYSTEM Grain size (micron G th Rotation -------6-t--;1

low rate u si rate (1'evs./ Load Rubbing 1n l'llbbld rubbed Composition (UL/hrs.) Inins.) (grns) body region region Eutectic. 0. 5 25 G0 Rotary {$332 11 0 l)o 1. a 75 400 Trans... is 11000 ""'{Long 1' 100!) Do. .75 850 400 iTmns. 1U

'lLong :,o0o 0 100 400 Win: brusl| {Eg?: :f 0 Do 1.0 13 60 Trans. 20 200 Long 20 2,000 9 0.- Pb-O.8 Sn-.. 1. 0 25 60 Rotary {'IIJga gs... 1,000

1 Transverse. Longitudinal.

have been performed using the wedge 21 and the rotary file 31 (rotated both with and against the cutting direction) as well as the wire brush 14 in such an apparatus. These trials have been i made using the eutectic systems Pb-Sn and Sn-Zn both at the eutectic composition and at compositions away from the eutectic. The motion used was simple rotation at rates between 25 and 850 revolutions per minute. Growth rates between one-half inch and 12 inches per hour have been used. Typical temperature gradients were 50 C. per centimeter. The loads applied to the rubbing body were varied between 60 and 400 grams. The average grain size produced in all of these trials varied between about 5 and 20 microns which is in the range of interest for superplasticity lAlden, Acta Met 15 (1967) 471]. There is no lamellar or dendritic substructure evident in these grains.

In the cases in which the rubbing body did not contact the whole interface area, the unrubbed areas immediately adjacent to the rubbed areas showed the typical grain size between 100 and 2,000 microns when looking at a section taken transverse to the growth direction. The grains, when looking at a longitudinal section, were typically greater than 2 millimeters long parallel to the direction of solidification. These grains showed the typical lamellar and dendritic substructures. These trials indicated that rubbing rate and growth rate were not a dominant factor in determining grain size. The trials indicate that the grain size is dependent on annealing time at temperatures near the solidification temperature so that faster growth rates and larger temperature gradients should yield smaller grain size or even amorphous materials. Superplastic elongation was demonstrated using materials so produced.

A trial was made using the Pb-Sn eutectic composition but producing a stirring action only with no contact between a typical lamellar eutectic structure was present. When the material included also a single-phase material such as Bi or Pb,

the grain size fell in the to 2,000 micron range. However, in the case of Bi, these grains were affected by the rubbing in that they were roughly equaxial and not elongated in the growth direction.

lclaim:

l. A method for the production of a crystalline solid selected from the group consisting of metal, semiconductor or organic compound from a liquid mass by unidirectional solidification characterized in that a nonreacting solid body is maintained in contact with and rubbed against at least a portion of the solidifying surface area of said crystalline solid during solidification at the interface between said crystalline solid and said liquid mass.

2. A method of claim 1 in which said crystalline solid is composed of a single-phase material.

3. A method of claim 1 in which said crystalline solid is composed of a multiphase material.

4. A method of claim 1 in which the rubbing action of said solid body is rotary in the plane of the said interface and over at least a major portion of the area of said interface.

5. A method of claim 4 in which the axis of rotation of the said rotary rubbing motion is caused to translate parallel to the said interface.

6. A method of claim 1 in which the rubbing action of said solid body is reciprocating in the plane of the said interface. 

2. A method of claim 1 in which said crystalline solid is composed of a single-phase material.
 3. A method of claim 1 in which said crystalline solid is composed of a multiphase material.
 4. A method of claim 1 in which the rubbing action of said solid body is rotary in the plane of the said interface and over at least a major portion of the area of said interface.
 5. A method of claim 4 in which the axis of rotation of the said rotary rubbing motion is caused to translate parallel to the said interface.
 6. A method of claim 1 in which the rubbing action of said solid body is reciprocating in the plane of the said interface. 